The present invention relates to a femoral stem for hip prosthesis used for replacing the natural joint of a hip joint with an artificial device. More particularly, the invention relates to a modified femoral stem to prevent subsidence.
It is well known that in the orthopaedic field, surgery of the hip joint is generally carried out to treat pathologies such as arthrosis, arthritis, hip luxation, femoral head and neck fractures, or similar conditions generating a progressive wear, pain, or dysfunction of the hip joint. The hip joint connects the femur to the pelvis and comprises the femur head engaged in the acetabulum which is a joint cavity on the outer face of the pelvic bone. In the femur, in addition to said femoral head, there is the neck connecting the head to the femoral body, having a rather longitudinal development.
On the basis of the natural shape of the hip, the prosthesis to be implanted comprises therefore of a femoral part and a pelvic part. The femoral part called a femoral stem is typically made of surgical grade metal, i.e., titanium alloy or stainless steel, and the pelvic part called acetabular cup is typically made of Ultra High Molecular Weight Polyethylene (UHMWP) and/or surgical grade metal, i.e., titanium alloy or stainless steel. The femoral stem comprises a main body also known as the stem, which is implanted into a longitudinal cavity of bone made in the natural femoral shaft. The acetabular cup is implanted into the bony bed made in the natural pelvic acetabulum. Such femoral stem ends at the top of the natural femoral shaft with a shoulder which blends into a femoral neck projecting from the shoulder and having a terminal cone. A spherical femoral head also made of metal or ceramics is inserted on to the above mentioned terminal cone and forms the pivot ball of the artificial ball and socket joint after placing it into the acetabular cup.
The surgical operation to the patient's hip begins with cutting the femoral neck to remove the femoral head and neck. Such operation is called a femoral neck resection or osteotomy. The resected natural femoral head and neck are removed to expose the top of the femoral canal. Subsequent preparation of the femur by proper pins, drills, and rasps prepares the internal canal of the femoral bone where the prosthetic stem will be inserted. The acetabulum is then prepared by removing cartilage and bone with spherical mill to form a seat into which the acetabular cup is then inserted. The femoral stem is then inserted into the prepared femoral canal. A spherical head is placed onto the terminal cone of the femoral prosthesis neck. After the femoral stem and head are assembled, the head is reduced into the implanted acetabular cup restoring the original configuration of the hip joint.
Once implanted, the femoral prosthesis and the acetabular cup recreate almost identically the original shape and function of the hip joint. Such provides pain relief and recovery of the joint function so as to allow the patient to have a normal life for many years.
The stem is anchored in the femur through three main methods. The first consisting of solidification of a liquid polymer around the stem that acts as a cement, connecting stem firmly to the bone. Most modern methods use a natural process whereby the femoral stem is press fit or compressed into the bony bed of the femoral canal. The direct contact of bone to stem allows for bone integration into the metal of the stem. The anchoring ability of the bone growth into the metal implant is enhanced by a roughened or textured outer surface of the metal. The final and less common method of stem fixation uses several screws that anchor the stem to the wall of the femoral canal.
One major disadvantage of press fit bony ingrowth method is subsidence. Subsidence is the progressive post-operative movement or migration of the implanted femoral stem down the bony canal of the femoral shaft. In certain patients, particularly active patients or animals that cannot understand instructions to minimize activity, the femoral stem may be impacted further into the femur prior to bony integration through repeated impacts from normal activity. Subsidence may result in: 1) a decreased ability of the bone to grow into the metal surface resulting in a loose femoral stem, 2) a significant change in position of the stem resulting in an increased incidence of dislocation of the femoral head out of the cup, and 3) fracturing of the femoral shaft as the implant gets pushed down the canal. Fracturing or breaking of the femur bone particularly in non-human patients, such as dogs or cats may be due to the thinner bone cortical walls of the bones compared to humans.
The present invention addresses these needs and provides other related advantages.